Carbon and oxygen fluxes from a small pond to the atmosphere:: Temporal variability and the CO2/O2 imbalance

Because of the relative strength of sediment processes compared to water column processes, natural and anthropogenic ponds represent an important component of the terrestrial hydrologic cycle and a site for recycling carbon to the atmosphere. Over 250 d of dissolved oxygen and carbon dioxide concentrations were measured on the 0.5 h timescale in a small Connecticut pond. Using approximately 8000 half-hour time intervals, Mirror Lake (Storrs, Connecticut) exhibits net annual fluxes for 2002 and 2003 of 80 and 86 mmol CO2 m(-2) d(-1) (+/- 26%) and 25 and -7.5 mmol CO2 m(-2) d(-1) (+/- 20%), respectively ( positive flux is to the atmosphere). The instantaneous ( 0.5 h) fluxes of both CO2 and O-2 to the atmosphere exhibit a standard deviation in the flux (measured every 0.5 h) of the order 100%, indicating a high degree of daily, weekly, and seasonal variability in the controlling processes, and the two gases rarely follow Redfield ratio stoichiometry. This net carbon flux to the atmosphere agrees with the range of carbon fluxes from various shallow inland aquatic waters. Using CO2 and O-2 mass balances, the minimum bacterial production of CO2 in the pond is estimated to be 100 and 81mmol CO2 m(-2) d(-1) for 2002 and 2003 (same order as the net CO2 flux from the pond), indicating the importance of the bacterial processes in pond carbon dynamics. Bacterial pathways that utilize NO3-1 or fermentation strongly favor CO2 production relative to O2 consumption and may provide a mechanistic explanation for the ( instantaneous to annual) CO2 to O-2 imbalance relative to the Redfield ratio. Thus, while natural and man-made ponds do provide advantages for flood control, sediment settling, and some degree of contaminant removal, ponds may also provide a locus for the processing of terrestrial carbon into a CO2 source to the atmosphere. Given the number of small ponds and their location in the hydrologic cycle, pond systems represent a coupling of hydrology and the carbon cycle worthy of greater investigation.